Show percentage in OSD when a progress bar is visible

The new design makes this visually simple, and in fact I think it
improves the composition to have the progrss bar anchored by dark-colored
elements on both sides.

OSDs without progress bars are unaffected.

BUG: 385602
FIXED-IN: 5.20

Merge Request: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/15